This invention relates generally to floating offshore structures, such as platforms, from which offshore operations, e.g., petroleum drilling and production, can be carried out and the riser support systems for use with these offshore structures. The invention is particularly concerned with riser support systems designed to support surface wellheads and associated equipment, usually on platforms floating in relatively deep water.
As hydrocarbon reserves decline, the search for oil and gas has moved offshore into increasingly deeper waters where economic considerations and physical limitations frequently militate against the use of platforms supported on the ocean or sea floor. Thus, most offshore drilling and production in deep water is conducted from floating platforms that support the drill rig and associated drilling and production equipment. The three types of floating platforms that see the most use in deepwater are tension leg platforms (TLPs), spars and semisubmersible platforms.
Tension leg platforms (TLPs) are moored to the ocean floor using semirigid or axially stiff (not axially flexible), substantially vertical tethers or tendons (usually a series of interconnected members). The TLP platform is comprised of a deck and hull similar in configuration and construction to the semisubmersible platform. The hull provides excess buoyancy to support the deck and to tension the tethers and production risers. The deck supports drilling and production operations. The use of axially stiff tethers tensioned by the excess buoyancy of the hull to moor the platform tends to substantially eliminate heave, roll and pitch motions, thereby permitting the use of surface wellheads and all the benefits that accompany their use.
Another type of floating structure used in offshore drilling and production operations is a spar. This type of structure is typically an elongated, vertically disposed, cylindrical hull that is buoyant at the top and ballasted at its base. The hull is anchored to the sea floor by flexible taut or catenary mooring lines. Although the upper portion of a spar's hull is buoyant, it is normally not ballastable. Substantially all the ballast is located in the lower portion of the hull and causes the spar to have a very deep draft, which tends to reduce heave, pitch and roll motions.
Semisubmersible floating platforms typically consist of a flotation hull usually comprising four or more large diameter vertical columns supported on two or more horizontal pontoons. The columns extend upward from the pontoons and support a platform deck. The flotation hull, when deballasted, allows the platform to be floated to the drill site where the hull is ballasted with seawater to submerge it such that the deck remains above the water surface. The platform is held in position by mooring lines anchored to the sea floor. Partially submerging the hull beneath the water surface reduces the effect of environmental forces, such as wind and waves, and large lateral column spacing results in small pitch and roll motions. Thus, the work deck of a semisubmersible is relatively stable. Although the semisubmersible platform is stable for most drilling operations, it usually exhibits a relatively large heave response to the environment because the pontoons are at a depth that exposes the structure to the rotational energy of large waves.
In order to use surface wells in floating offshore platforms or hulls that are subject to pitch roll and heave motions, such as the semisubmersible and spar platforms described above, the surface wellheads typically must be supported by top tensioning systems and/or individually buoyant risers. Typically, hydraulic top tensioning systems are also required to support risers in TLPs. Top tensioning systems, such as hydraulic cylinder assemblies, add extra weight to the hull supporting the platform, are mechanically complex and add significantly to costs. Individually buoyant risers are relatively complex and expensive subsystems, and the individual buoyancy cans used in these subsystems require significant lateral support and have a large number of moving parts that require close fits and/or a large number of wear or centralizing mechanisms. Thus, the use of individual buoyancy cans results in a large well bay size and increased overall hull size.
It is clear from the above discussion that conventional riser systems needed to support surface wellheads in floating offshore platforms used in deepwater exploration and production have significant disadvantages. Thus, there exists a need for other riser support systems that are mechanically simple and relatively inexpensive for use in these offshore systems.